Multilayer inductor

ABSTRACT

A multilayer inductor in which a DC resistance is small so that a high direct current can be applied thereto, has a structure in which a plurality of magnetic layers is stacked and coil conductor patterns are provided between the magnetic layers. The coil conductor patterns are spirally connected to each other via through-holes formed in the magnetic layers. The coil conductor patterns are arranged such that the areas of projected planes of the coil conductor patterns on main surfaces of the magnetic layers are within a range from about 35% to about 75% of the areas of the main surfaces of the magnetic layers.

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to multilayer inductors. Moreparticularly, the present invention relates to multilayer inductors suchas choke coils for DC/DC converters.

[0002] 2. Description of the Related Art

[0003] In a DC/DC converter used for a main power source of a personalcomputer, it is necessary to provide a coil or an inductor having asmall DC resistance such that a high direct current can be appliedthereto. Conventionally, these inductors are defined by windingdrum-shaped cores with conductive wires.

[0004]FIG. 10 illustrates an example of a conventional inductor. In theinductor 1 shown in FIG. 10, a drum-shaped core 2 having a circularcross-section is wound with a conductive wire 3.

[0005]FIG. 11 illustrates another example of a conventional inductor. Inthe inductor 1 shown in FIG. 11, a drum-shaped core 2 having arectangular cross-section is wound with a conductive wire 3.

[0006]FIG. 12 illustrates another example of a conventional inductor. Inthe inductor 1 shown in FIG. 12, a drum-shaped core 2 having arectangular cross-section is wound with a conductive wire 3. Inaddition, an air gap or a cavity 4 is provided in the center of the core2, that is, in the central portion of a coil defined by the conductivewire 3.

[0007] In the inductor 1 shown in FIG. 11, unlike the inductor 1 shownin FIG. 10, the conductive wire 3 has a rectangular cross section. As aresult, since the entire space where the conductive wire 3 is wound iseffectually used with no clearance or gaps, the DC resistance is reducedand a high direct current is thereby applied to the inductor.

[0008] Additionally, unlike the inductor 1 shown in FIG. 11, in theinductor 1 shown in FIG. 12, the air gap or the cavity 4 is provided inthe center of the core 2, that is, in the central part of the coildefined by the conductive wire 3 such that a magnetic flux is cut off.With this arrangement, the DC application characteristics of inductanceare improved.

[0009] However, in the inductor 1 shown in each of FIGS. 10 to 12, it isimpossible to simultaneously produce the core 2 which is made of ferriteand the inside wire 3. In this case, for example, an E-shaped core iswound with a conductive wire, on which another E-shaped core isdisposed. Thus, the cores are not in close surface-contact with eachother, thereby causing characteristic deterioration and variations.Furthermore, the process for manufacturing the core is complicated.Additionally, since the cores are molded and the conductive wire havinga rectangular cross section is expensive, production costs increase.

[0010] To overcome the above-described problems, other conventionalinductors have been made. For example, multilayer inductors aredisclosed in Japanese Unexamined Patent Application Publication No.10-12443 and Japanese Unexamined Patent Application Publication No.10-27712.

[0011]FIG. 13 illustrates an example of a conventional multilayerinductor. A multilayer inductor 5 shown in FIG. 13 includes a multilayerstructure 6. The multilayer structure 6 includes a plurality of stackedmagnetic layers 6 a, between which coil conductor patterns 7 areprovided. The coil conductor patterns 7 are spirally connected to eachother via through-holes provided in the magnetic layers 6 a. Inaddition, external electrodes 8 a and 8 b are provided at the ends ofthe multilayer structure 6. The external electrodes 8 a and 8 b areconnected to the ends of a coil defined by the coil conductor patterns7. Furthermore, to improve the DC application characteristics ofinductance, a cavity 9 a is provided in the center of the multilayerstructure 6 or the magnetic layers 6 a, that is, in the central portionof the coil.

[0012]FIG. 14 illustrates another example of a conventional multilayerinductor. In a multilayer inductor 5 shown in FIG. 14, unlike theinductor 5 shown in FIG. 13, the center of the multilayer structure 6 orthe magnetic layers 6 a, that is, the central portion of the coil isformed of a nonmagnetic ceramic 9 b.

[0013] In the multilayer inductor 5 shown in each of FIGS. 13 and 14, incontrast with the inductors 1 shown in FIGS. 10 to 12, manufacturing issimplified such that production costs are reduced.

[0014] In the above-described conventional multilayer inductors,however, since the areas of coil conductor patterns are reduced, the DCresistance is increased. As a result, it is impossible to apply highdirect currents to the inductors.

SUMMARY OF THE INVENTION

[0015] To overcome the above-described problems with the prior art,preferred embodiments of the present invention provide a multilayerinductor having a greatly reduced DC resistance such that a high directcurrent can be applied thereto.

[0016] According to one preferred embodiment of the present invention, amultilayer inductor includes a plurality of stacked magnetic layers,through-holes provided in the magnetic layers, and a plurality of coilconductor patterns disposed between the plurality of magnetic layers andspirally connected to each other via the through-holes. In thismultilayer inductor, the area of a projected plane of a circuit of eachcoil conductor pattern on a main surface of each of the magnetic layersis in a range from about 35% to about 75% of the area of the mainsurface of the magnetic layer.

[0017] In addition, in this multilayer inductor, a nonmagnetic portionis provided in the vicinity of the coil conductor patterns in themagnetic layer.

[0018] In the multilayer inductor according to various preferredembodiments of the present invention, the area of the projected plane ofa circuit of each coil conductor pattern preferably ranges from about35% to about 75% of the area of the main surface of each magnetic layer.With this arrangement, DC resistance of the coil defined by theplurality of coil conductor patterns is greatly reduced, andaccordingly, a greatly increased direct current can be applied to thecoil.

[0019] Furthermore, in the above-described multilayer inductor, thenonmagnetic portion is provided in the vicinity of the coil conductorpatterns in the magnetic layer. As a result, a magnetic flux is cut offat the nonmagnetic portion. Thus, since magnetic saturation does notoccur near the coil defined by the plurality of coil conductor patterns,the DC application characteristics of inductance are greatly improved.

[0020] Other features, steps, processes, characteristics, and advantagesof the present invention will become more apparent from the detaileddescription of preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 illustrates a multilayer inductor according to a preferredembodiment of the present invention.

[0022]FIG. 2 is an exploded perspective view of the multilayer inductorshown in FIG. 1.

[0023]FIG. 3 is a plan view showing a main surface of each magneticlayer and a projected plane of each coil conductor pattern of themultilayer inductor shown in FIG. 1.

[0024]FIG. 4 is a perspective view showing another configuration ofmagnetic layers and coil conductor patterns.

[0025]FIG. 5 is a graph showing electrical characteristics of amultilayer inductor using the magnetic layers and the coil conductorpatterns shown in FIG. 4.

[0026]FIG. 6 illustrates a multilayer inductor according to anotherpreferred embodiment of the present invention;

[0027]FIG. 7 is an exploded perspective view of the multilayer inductorshown in FIG. 6.

[0028]FIG. 8 is a graph showing electrical characteristics of themultilayer inductor obtained where no cavity is provided, where a cavityis formed, and where the cavity size is increased.

[0029]FIG. 9 is an exploded perspective view of a multilayer inductoraccording to another preferred embodiment of the present invention.

[0030]FIG. 10 illustrates a conventional inductor.

[0031]FIG. 11 illustrates another conventional inductor.

[0032]FIG. 12 illustrates another conventional inductor.

[0033]FIG. 13 illustrates another conventional inductor.

[0034]FIG. 14 illustrates another conventional inductor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0035]FIG. 1 illustrates a multilayer inductor according to a preferredembodiment of the present invention. FIG. 2 is an exploded perspectiveview thereof. A multilayer inductor 10 shown in each of FIGS. 1 and 2includes a multilayer structure 12.

[0036] The multilayer structure 12 includes a plurality of stackedmagnetic layers 14. Between the magnetic layers 14 first coil conductorpatterns 16 a, second coil conductor patterns 16 b, and lead-out coilconductor patterns 16 c and 16 d are provided. In this case, theplurality of first coil conductor patterns 16 aand the plurality ofsecond conductor patterns 16 b are alternately provided. In FIGS. 1 and2, some of the plurality of first coil conductor patterns 16 aand theplurality of second conductor patterns 16 b are not shown to avoidrepetition. The lead-out coil conductor pattern 16 c is provided on thetop of the first and second coil conductor patterns 16 aand 16 b. Thelead-out coil conductor pattern 16 d is provided thereunder. Thelead-out coil conductor pattern 16 c has a lead-out portion extending toone end of the magnetic layer 14. In addition, the other lead-out coilconductor pattern 16 d has a lead-out portion extending to the other endof the magnetic layer 14. Furthermore, through-holes 18 are provided inthe magnetic layers 14 disposed between the lead-out coil conductorpatterns 16 c and 16 d. The coil conductor patterns 16 a, 16 b, 16 c,and 16 d are spirally connected to each other via the through-holes 18.

[0037] In the multilayer inductor 10, as shown in FIG. 3, the coilconductor patterns 16 a, 16 b, 16 c, and 16 d are provided such that thearea Sc of a projected plane of a circuit on a main surface of eachmagnetic layer 14 ranges from about 35% to about 75% of the area Sm ofthe main surface of the magnetic layer 14.

[0038] Furthermore, external electrodes 20 a and 20 b are provided onthe ends of the multilayer structure 12. The external electrodes 20 aand 20 b are connected to the lead-out portions of the coil conductorpatterns 16 c and 16 d, that is, to the ends of a coil defined by thecoil conductor patterns 16 a, 16 b, 16 c, and 16 d.

[0039] To manufacture the multilayer inductor 10, for example, first,each coil conductor pattern is printed on a green sheet used as eachmagnetic layer, by a method such as screen printing. Then, afteralternately stacking green sheets having the first coil conductorpatterns provided thereon and green sheets having the second coilconductor patterns provided thereon, green sheets having the lead-outcoil conductor patterns provided thereon are disposed on the top of thestacked sheets and at the bottom thereof. Next, on the top of the entirestacked sheets and at the bottom thereof, further green sheets areprovided to produce a multilayer structure. After the multilayerstructure is pressed and fired, the external electrodes are disposedthereon to produce the multilayer inductor 10.

[0040] In the multilayer inductor 10, the area Sc of the projected planeof the circuit of the coil conductor patterns 16 a, 16 b, 16 c, and 16 dis preferably within the range of about 35% to about 75% of the area Smof the main surface of the magnetic layers 14. As a result, DCresistance of the coil formed by the coil conductor patterns 16 a, 16 b,16 c, and 16 d is greatly reduced and a high direct current is appliedthereto.

[0041] When the ratio of the area Sc of the projected plane of each coilconductor pattern with respect to the area Sm of the main surface ofeach magnetic layer is less than about 35%, the DC resistance of thecoil is increased, which is not preferable. On the other hand, when thearea ratio of the same is more than about 75%, a magnetic flux does notpass through the coil, with the result that the obtained inductance isundesirably reduced.

[0042] Since the multilayer inductor 10 can be manufactured by theabove-described stacking method, the process for manufacturing is not socomplicated as that for manufacturing inductors obtained by windingwires, and cost of production is thereby greatly reduced.

[0043] Furthermore, the multilayer inductor 10 manufactured byintegrating the components can be easily made thinner.

[0044] Now, a description will be provided of the electricalcharacteristics of the above multilayer inductor 10. In this case, themultilayer inductor 10 preferably includes disk-shaped magnetic layers14, and coil conductor patterns 16 a, 16 b, 16 c, and 16 d having asubstantially ring-shaped projected plane of the circuit of the coilconductor patterns on the main surfaces of the magnetic layers 14.

[0045] For example, as shown in FIG. 4, the diameter D of eachdisk-shaped magnetic layer 14 is preferably about 4 mm. In addition, ineach of the coil conductor patterns 16 a, 16 b, 16 c and 16 d, a centersection C in the width direction of the projected plane of the circuitof the conductor pattern has a substantially circular with a diameter ofabout 2 mm in length. The width W of each of the coil conductor patterns16 a, 16 b, 16 c, and 16 d is preferably about 1 mm. In this case, thearea of the main surface of each magnetic layer is approximately 12.56mm, and the area of the projected plane of a circuit of each of the coilconductor patterns 16 a, 16 b, 16 c, and 16 d is approximately 6.28 mm².Thus, the ratio of the area of the projected plane thereof with respectto the area of the main surface of the magnetic layer 14 is about 50%.

[0046] In this example, when a multilayer inductor having a height or athickness of about 1 mm is manufactured, with respect to an inductanceof 10 μH, the value of a DC resistance is approximately 0.2 Ω.

[0047] In addition, when the width W of the coil conductor pattern isabout 0.3 mm, the area ratio is about 15%. In this case, the necessarynumber of turns of the coil conductor pattern to obtain the sameinductance of 10 μH is reduced, and the obtainable maximum inductance ismuch larger. A DC resistance with respect to the inductance of 10 μH isincreased to be approximately 0.4 Ω.

[0048] Table 1 shows the value of a DC resistance with respect to eachinductance obtained when the width W of the coil conductor pattern ischanged in the above example. TABLE 1 ELECTRODE WIDTH (mm) 0.3 0.5 0.71.0 1.2 1.5 AREA RATIO (%) 15 25 35 50 60 75 DC  5 μH 0.25 0.18 0.140.12 0.11 0.11 RESISTANCE 10 μH 0.39 0.28 0.22 0.20 0.19 0.18 (Ω) 20 μH0.57 0.41 0.33 0.30 0.29 — 30 μH 0.80 0.56 0.44 — — — 50 μH 1.04 0.82 —— — —

[0049] Segments which have no DC resistance value in the Table 1indicate cases in which the values are not available.

[0050]FIG. 5 is a graph for illustrating the content of Table 1. InTable 1 and the graph shown in FIG. 5, obviously, when the width W ofthe coil conductor pattern is increased, the DC resistance is reduced.However, the reduction ratio becomes gradually smaller, and the effectsdue to increases in the area ratio are reduced. In addition, eachobtainable maximum inductance is reduced.

[0051] Furthermore, increasing the width W of the coil conductor patternallows the DC resistance to be smaller. However, in the above example,when considering the range of obtainable inductance, in a range betweenabout 5 μH and about 30 μH, the obtainable area ratio is preferablyabout 35% or greater.

[0052]FIG. 6 illustrates a multilayer inductor according to anotherpreferred embodiment of the present invention. FIG. 7 is an explodedperspective view of the multilayer inductor. In a multilayer inductor 10shown in each of FIGS. 6 and 7, unlike the multilayer inductor 10 shownin each of FIGS. 1 and 2, an air gap or a cavity 22 is disposed inside asingle second coil conductor pattern 16 b.

[0053] The multilayer inductor 10 shown in each of FIGS. 6 and 7 ispreferably manufactured in the same way as the multilayer inductor 10shown in each of FIGS. 1 and 2 is manufactured. However, when the cavity22 is formed, for example, after an organic paste such as carbon isthinly applied inside the second coil conductor pattern on a greensheet, the entire structure is fired.

[0054] In the multilayer inductor 10 shown in each of FIGS. 6 and 7,unlike the multilayer inductor 10 shown in each of FIGS. 1 and 2,particularly, since the cavity 22 cuts off a magnetic flux passingthrough the approximate center of the coil, magnetic saturation hardlyoccurs at the approximate center of the coil. As a result, good DCapplication characteristics of inductance can be obtained.

[0055] The size and position of the cavity 22 can be easily changed bychanging the thickness of the applied organic paste and the position inwhich the organic paste is applied. With this arrangement, requiredcharacteristics can be obtained.

[0056]FIG. 8 is a graph showing the electrical characteristics of themultilayer inductor obtained when no cavity is formed, when a cavity isformed, and when the size of the cavity is increased. As evident in thegraph shown in FIG. 8, DC application characteristics obtained when thecavity is formed are better than those obtained when no cavity isformed. Furthermore, it is seen that when the size of the cavity isincreased, the inductance DC application characteristics of themultilayer inductor are even more improved.

[0057] Instead of forming the cavity 22 by applying an organic paste,when a resin sheet having the same size as that of the area where theorganic paste is applied is disposed, this is equivalent to a situationin which a nonmagnetic part is formed in the vicinity of the coilconductor patterns. Thus, the magnetic flux is cut off at thenonmagnetic part. As a result, since magnetic saturation hardly occursnear the coil, the DC application characteristics of inductance aregreatly improved.

[0058]FIG. 9 is an exploded perspective view of a multilayer inductoraccording to another preferred embodiment of the present invention.Unlike the multilayer inductor 10 shown in each of FIGS. 1 and 2, in amultilayer inductor 10 shown in FIG. 9, particularly, first and secondcoil conductor patterns 16 a and 16 b have substantially C-shapedconfigurations, and lead-out coil conductor patterns 16 c and 16 d havesubstantially J-shaped configurations. As shown here, even with coilconductor patterns having different configurations, the same advantagescan be obtained.

[0059] As described above, in the multilayer inductor according tovarious preferred embodiments the present invention, since the DCresistance is small, a high direct current can be applied thereto.

[0060] Furthermore, in the multilayer inductor, when the nonmagneticpart is disposed near the coil conductor patterns of the magneticlayers, the DC application characteristics of inductance are greatlyimproved.

[0061] While preferred embodiments have been described above, variationsthereto will occur to those skilled in the art within the scope of thepresent invention. The scope of the invention is therefore to bedetermined solely by the appended claims.

What is claimed is:
 1. A multilayer inductor comprising: a plurality ofstacked magnetic layers; through-holes formed in the stacked magneticlayers; and a plurality of coil conductor patterns disposed between theplurality of magnetic layers and spirally connected to each other viathe through-holes; wherein the area of a projected plane of a circuit ofeach coil conductor pattern on a main surface of the magnetic layer isin a range from about 35% to about 75 of the area of the main surface ofthe magnetic layer.
 2. A multilayer inductor according to claim 1 ,further comprising a nonmagnetic element disposed in the vicinity of thecoil conductor patterns in the magnetic layer.
 3. A multilayer inductoraccording to claim 1 , further comprising external electrodes providedon the ends of the multilayer inductor.
 4. A multilayer inductoraccording to claim 3 , wherein the coil conductor patterns include leadout portions which are connected to respective ones of the externalelectrodes.
 5. A multilayer inductor according to claim 1 , wherein themagnetic layers are substantially disk-shaped.
 6. A multilayer inductoraccording to claim 1 , wherein the projected plane of the circuit of thecoil conductor patterns on the main surfaces of the magnetic layers issubstantially ring-shaped.
 7. A multilayer inductor according to claim 1, wherein an air gap or a cavity is disposed inside one of the pluralityof coil conductor patterns.
 8. A multilayer inductor according to claim1 , wherein selected ones of the plurality of coil conductor patternshave substantially C-shaped configurations.
 9. A multilayer inductoraccording to claim 1 , wherein selected ones of the plurality of coilconductor patterns have substantially J-shaped configurations.